<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>2-6 border</title>
  <style>
    .box {
      width: 0;
      height: 0;
      /* 左上 */
      border-left: 100px solid #00a2e8;
      border-top: 100px solid #ed1c24;
      /* 右下同样的效果 */
      /* border-right: 100px solid #ff7f27;
    border-bottom: 100px solid #22b14c; */
    }
  </style>
</head>

<body>
  <div class='box'></div>
</body>

</html>